libobs/util: Fix C++ compilation issue

When the C header circlebuf.h is used from a C++ source file, this
implicit cast from void to uint8_t* will cause an error unless changed
to an explicit cast.

libobs/util/circlebuf.h

@@ -256,7 +256,7 @@ static inline void circlebuf_pop_back(struct circlebuf *cb, void *data,
 
 static inline void *circlebuf_data(struct circlebuf *cb, size_t idx)
 {
-	uint8_t *ptr = cb->data;
+	uint8_t *ptr = (uint8_t*)cb->data;
 	size_t offset = cb->start_pos + idx;
 
 	if (idx > cb->size)